The retinal pigmented epithelium (RPE) lies at the interface between the choroid and the neural retina, where it plays a central role in the physiology of the eye. Because the RPE interacts with two different tissues, its plasma membrane is differentiated into basolateral (facing the choroid) and apical (facing the neural retina) membranes. Each membrane has its own protein composition, reflecting its unique functions. To achieve this polarity, plasma membrane and secretory proteins must express sorting signals that direct them specifically to one or the other membrane. Chicken embryo RPE will be cultured on filters as intact monolayers, allowing the membrane and secretory proteins of the apical membrane to be analyzed independently of those of the basolateral membrane. The polarity of secretion of endogenous secretory proteins will be catalogued, while recombinant DNA technology will be used to express exogenous proteins from transfected cDNA's, in order to probe the sorting mechanism. Model secretory proteins that lack sorting signals will be expressed to determine whether the default (signal independent) transport pathway leads to one or both plasma membranes. Model membrane glycoproteins will be used to compare the sorting mechanism employed by RPE with sorting mechanisms employed by other epithelia. Metabolically-labeled secretory proteins will be analyzed by gel electrophoresis of the apical or basolateral media compartments. When appropriate, specific proteins will be studied by immunoprecipitation of the medium. The transport of model proteins will be followed by immunobiochemical and immunocytochemical techniques.